In wireless communications standards individual wireless devices often send information back and forth between each other in the form of discrete frames sent in wireless signals. Each of these frames contains some information to be passed, as well as some information to allow the receiving device to properly receive and decipher the information in the frame.
Because of differences in local clock operation and variances in signal transmission paths, it is generally necessary for a receiving device to synchronize the phase of an internal clock with the phase of a received signal before the signal can be processed. In many implementations a frame will include a preamble that is placed at the beginning of the frame which allows the receiving device an opportunity to synchronize with the incoming frame. This is often called acquiring or locking onto the signal containing the incoming frame.
A preamble is generally a known, recognizable, and repeated pattern of bits that the receiving device can look for. This pattern is often generated by a formula known to both the transmitting device and the receiving device, and which can be easily detected.
In order to successfully identify the preamble, the receiving device must operate using a local clock that is synchronized with the phase of the incoming signal. As a result, in attempting to lock onto the incoming signal, the receiving device will generally vary the phase of its local clock, attempting to find a phase at which it can successfully detect the bit pattern in the preamble. Once the receiving device successfully identifies the preamble, i.e., recognizes the bit pattern being sent in the preamble, it will have successfully synchronized its local clock with the phase of the received data frame, and will have locked onto the bit pattern. In a wireless device there are generally several levels of synchronization. A device can synchronize to an oscillator frequency of the incoming signal, to a symbol or chip being sent in the incoming signal, or to a series of bits being sent in the incoming signal. Generally a device will have to sequentially synchronize on increasing levels of the signal, building upon the synchronization with the lower levels.
In implementation, most devices that use preambles do not initialize the formula (e.g., the polynomial) used for generating the preamble to the same initial conditions in every frame. In other words, while a preamble will generally contain a known and repeated bit pattern, the start of that bit pattern will be essentially random with respect to the start of the preamble. As a result, once the receiving device successfully synchronizes with an incoming preamble, it has no way of knowing how much time remains before the preamble ends.
In a narrow band system, a receiver can use a carrier (i.e., energy) detection to determine when a preamble starts, and thus how much time remains. In a UWB system, however, the signals have low signal-to-noise (SNR) ratio, meaning energy detection is generally an undesirable solution.
This can be a problem in certain devices that require additional signal processing or receiver preparation before receiving information from a frame. For example, some devices may perform operations on an incoming signal to improve signal quality. These operations can tale the form of linear equalization, decision feedback equalization (DFE), fine automatic gain control (AGC), and/or the use of RAKE receivers. These processes take a certain amount of time to train before they are ready to operate. And since the receiving UWB device does not know how much time remains in the preamble after signal lock, it cannot determine whether there is sufficient time remaining for receiver training, AGC refinement, signal normalization, or the like.
This can be a problem because if the receiving device starts training and the preamble ends before the training is completed, the receiving device may not be able to successfully receive the incoming data without an unacceptable number of errors. This can also be a problem in embodiments that continue to refine acquisition (e.g., using multiple acquisition fingers). In this situation, since the receiver doesn't know when the preamble will end, it doesn't know if it has sufficient time to try and look for a better acquisition lock before it must start training. It then runs the risk of either wasting too much time refining acquisition lock so that it has insufficient time for training, or it might stop refining acquisition too early in an effort to make certain it will have enough time for training.
Accordingly, it would be desirable in the art for a solution to the problems associated with unknown relative signal lock timing, and further to the problems associated with trying to allow adequate time for receiver training when a receiving device has no way of knowing the remaining time in a preamble once signal lock is completed.